GT3788VA Variable nozzle turbojet build

[mitch]

I just realized today that my "boost" pressure gauge tubing had a hole in it during the run, meaning the mechanical gauge was reading much lower than what the actual "boost" pressure was. It was a pretty decent sized hole, so I would have to assume that actual pressure was quite a bit higher than what was being read, maybe around 8 or 10 psi.

[mitch]

Had another 2 successful runs of the engine tonight, I maxed out the fuel pump with the injector I have now, won't go above 6 or 7 ish psi p2. Runs great up to those lower power levels though, managed to hook up a thermocouple and read a max egt of about 900-925° fahrenheit, which seems a bit low, but is definitely on the safe side.
I took video, but it wasn't too exciting.
One thing I am concerned about however, is the oil consumption. I think the turbo is slowly losing an oil seal, the air inlet tube has a fine oil coating the inside of it, and the turbo smokes once it is shut down. The combustion chamber is also decently covered in soot internally, not sure if this is from the burning oil or the lower power levels I am running at recently. Turbine wheel has oil on the tips of it after allowing the engine to cool down using a leaf blower. I think I am in need of a new turbo oil seal, any thoughts from anyone else?

[racket]

Hi Mitch

Temps sound OK , they increase at higher P2s and when you get the right jet nozzle size on her.

Soot in the combustor is "normal".

If you have the oilpump operating and the engine not running , there will be some oil leakage past the AIR SEALS , they aren't oil seals , the piston ring seals are to stop air/gases getting into the bearing cavity.

Everything sound normal to me ;-)

Cheers
John

[mitch]

Mar 12, 2017 2:30:15 GMT -5 racket said:

Hi Mitch

Temps sound OK , they increase at higher P2s and when you get the right jet nozzle size on her.

Soot in the combustor is "normal".

If you have the oilpump operating and the engine not running , there will be some oil leakage past the AIR SEALS , they aren't oil seals , the piston ring seals are to stop air/gases getting into the bearing cavity.

Everything sound normal to me ;-)

Cheers
John

Racket,

That makes sense about the seals, I'll keep that in mind!

Also, for anyone interested, I am doing a project for my engine airflow class, where I am flowing my turbine's combustion chamber with modifications made to the angle of the inlet tube. 
I am doing the testing on a Superflow flow bench, used for testing piston engine cylinder heads to find swirl and flow numbers at different test pressures. 
Today I tested the combustion chamber with my current inlet tube setup (which produces a lot of swirl due to the positioning and angle of the inlet tube) and found that at 28" H20, my combustion chamber flowed 304 cubic feet of air per minute. 
I am going to modify the inlet tube to the combustion chamber to decrease swirl and hopefully increase penetration into the flame tube, and take egt readings at several different P2 pressures for comparison to the swirling inlet. 

[racket]

Hi Mitch

Interesting flow number.............I can't remember hearing of anyone doing it before, look forward to seeing how changes go

Cheers
John

[mitch]

I'll be sure to post all of my findings on here!

Ran the engine again today, up to 15 PSI p2, amazing how loud it is at that power level. I noticed the throttling of the engine was a bit less smooth and responsive with the bigger liquid fuel injector, but it still ran well. Turbine outlet temperature at 10 PSI was 1060 degrees F. It ran for probably 4 minutes above 8 PSI p2. I tried to throttle it up to 15 PSI p2 again, and it ruptured one of the rubber couplers that I connect the compressor outlet to the combustion chamber with! This was to be expected though, the combination of radiant heat from the turbine housing and combustion chamber, along with the elevated operating pressures led to conditions that those connectors are not designed for. Other than that and some minor oil pump issues (a weld on the pulley broke) the engine ran great again. I will probably get some of the "Vibrant" brand rubber connectors, as they are made for high temp high pressure conditions on performance piston engines.
Oil temps stayed safe, still under 120 degrees with no cooling fan on the oil cooler.

[racket]

Hi Mitch

I always used radiator type hose for my flexible connection , it only needed a very short length piece of ~2" as the metal bits inside were within a 1/4" of each other .

LOL, 15 psi is just starting to get noisy ..............it improves ;-)

Cheers
John

[mitch]

Hi all,
A couple quick questions for anyone reading.

As for my airflow project using the combustion chamber, I need some ideas/thoughts.

I need to show the difference between the airflow characteristics of a combustion chamber that uses a tangential inlet tube, which creates a large amount of swirl, compared to an inlet tube that discharges straight into the chamber, and does not create much swirl at all. Would the pressure inside the flame tube be greater without the inlet air swirling around inside the chamber? I am trying to think of a measurable way to show differences inside of the chamber with large amounts of swirl motion and with minimal amounts of swirl motion.
Any ideas would be appreciated

[mitch]

I came up with a solution to this, using a small computer fan mounted inside the chamber, which spins, then produces a voltage based on it's rpm, which I measure with a multimeter

I do have a question that I cannot seem to figure out though.

How come when my turbocharger is used in gas turbine form, it produces so little power compared to the turbo being used on the 6.0 liter diesel it came off of? For example, the engine should make about 50 lbs of thrust according to jetspecs, which in shaft hp form, will be less than 50 hp (which is still a good amount of power for a small go kart or bike). However, the diesel piston engine with the same turbo attached to it, is capable of producing 500 hp and well over 600 lb ft of torque.
Why is this? I realize there probably isn't a short answer, but it has been something I have been thinking about lately. I assume it could have something to do with the fact that not all of the air that is pushed through a gas turbine gets burned, and a good amount of it is used for cooling?

[racket]

Hi Mitch

Yep , less fuel burned , plus the compression ratio ,......... at 3:1 PR even with max temp of say 900 C theres only a ~250 C drop with exhaust temps of 650 C , but your diesel has a much higher pressure ( compression) ratio , or to be more precise, a greater expansion ratio with much greater temperature drop , several times more , so more energy extracted which means more power.

Yeh , its kinda disappointing that we get so little power from our engines , but its the same for all low pressure ratio turbine engines , ..........when I first got my TV84 engine running and found I only had 100 lbs of thrust I thought I was doing something wrong as surely it should produce more from all that fuel being burned and the massive amounts of air going through it ........LOL, years down the track , and after learning to do the maths, I found out there simply wasn't any way it could produce more power .

With the freepower added to the TV84 it produced 115 RWH on the bike dyno , which wasn't too bad considering the turbo was off a 475 HP Detroit Diesel truck engine .

Cheers
John

[enginewhisperer]

yep, unfortunately most of the air is just cooling!

[mitch]

Hi all,

Quick update on the gas turbine, and the results from my airflow project testing on the flow bench.

I found that with the new inlet design, the chamber flows 12 CFM more than the with the previous inlet tube. The tests were both done at 28" H2O, the chamber with the original "swirling" inlet flowed 304 CFM, while the chamber with the new inlet flowed 316 CFM. 

Another thing that I found to be interesting, was the in-chamber airflow characteristics. Measuring swirl inside of the chamber was very difficult to do, as most devices available to me in our lab are for measuring the flow characteristics produced by the ports and valves on piston engine heads. I ended up looking at the pressure differences inside of the chamber itself to get an idea of what the air is doing once it enters the chamber. I looked specifically at the pressure difference between the area where the air enters the chamber (between the chamber wall and flametube wall), and directly inside the flametube (at the base just below the primary hole zone). I found that with the original inlet tube, there was a 2" H2O difference (with a 28" test pressure), showing that the air pressure was higher outside of the flametube than inside the flametube. After testing with the new inlet tube, I found that there was no pressure difference at all. 

I haven't had time to make a test run with the new chamber yet, but I am hoping I can later on today. 

I cut the old inlet tube off of the combustion chamber. The original tube produced a lot of swirl inside of the chamber, between the chamber wall and flametube, due to the angle it came into the tube at. 



Extending and modifying the inlet tube hole to match up with the new inlet tube





The new inlet tube welded onto the combustion chamber. The new inlet is larger in volume than the previous, and should produce more "tumbling" of the inlet air than the previous. 





The chamber mounted on the flow bench, ready to be flow tested

[racket]

Hi Mitch

Some interesting numbers there :-)

Yep , that 2" over 28" is ~7% , too much for a generously sized combustor , we should have <5% , your negligible difference is very good , better than expected actually , though maybe once huge amounts of air are going through the combustor there'll be an increase in drop .

The pressure drop following the previous swirling tangential delivery was expected due to the usual laws of physics , and would have been much greater once the engine was running :-(

It would have been interesting to see if there was any sort of radial variation in static pressures across the flametube with the tangential delivery , theoretically there should have been as the air "spiraled into the vortex "

Keep those numbers coming :-)

Cheers
John

[mitch]

A quick update,
I haven't had much time to work on the engine lately, as I am rebuilding a 13b turbo engine for my Mazda, as my current NA 13b blew over the summer.

However, here is a quick video of the last run of the turbojet, which ran nicely at 10 psi compressor outlet!
www.youtube.com/watch?v=2FbWnlQGs1s&feature=youtu.be

Also some pics of my 13b rebuild, in case anyone is interested:

[mitch]

Haven't had much time lately to work on the engine, but I have been thinking about a few modifications I would like to make based on some recent runs I made with the engine.

I was doing some test runs using a 12 GPH nozzle, which is the largest injector I have for the engine. I found that the nozzle flow rate means that the spray pattern and fuel atomization is poor at lower fuel pressures, which are found during idle and low compressor outlet pressures on my engine. This makes the injector drip and miss my evaporator tubes, resulting in poor running conditions ( flames out the turbine housing, high egts, lots of smoke on cooldown).

To combat this, I want to take my horizontally mounted combustion chamber and mount it vertically, which will allow fuel to drip into the evap tube due to gravity, which should mostly get rid of my problems.

Am I overlooking anything here, or will this idea seem to work?